The present invention relates generally to devices and methods for measuring time delay and chromatic dispersion, and more particularly to devices and methods for accurately measuring the time delay and chromatic dispersion in a component by use of optical signals hose amplitudes are modulated at two or more modulation frequencies.
A conventional method for measuring chromatic dispersion in fibers is to amplitude modulate the output of a tunable single wavelength laser. As the laser frequency is tuned, the phase shift on the AM modulation changes in response to the dispersion in the fiber. In the Fourier or frequency domain, the amplitude modulated signal has two AM sidebands. The phase difference between the two AM sidebands is proportional to a delay in the fiber. The delay measured at various optical frequencies in the fiber is then used to determine the chromatic dispersion of the fiber. This method for measuring chromatic dispersion works adequately for measuring fiber dispersion where the dispersion changes are small relative to the spacing in frequency of the two AM sidebands. It also works adequately when there is no differential attenuation in the two AM sidebands.
When making a conventional chromatic dispersion (CD) measurement of a long fiber, which is the frequency derivative of a time delay, two conditions are normally assumed. The first assumption is that the fiber dispersion does not change rapidly in the frequency range defined by the spacing of the AM sidebands. This assumption allows the differential phase measurement sampled at the frequency spacing between the two sidebands to accurately approximate a derivative operation. The second assumption is that there in no amplitude filtering of the input signal. This second assumption allows the relative magnitudes for the two AM sidebands to remain unchanged after passing through the fiber. When measuring the chromatic dispersion of components that have optical characteristics that change rapidly depending upon optical frequency, however, both the magnitude and phase can vary quickly relative to the spacing of the AM sidebands, which causes errors in the dispersion measurement.
This conventional measurement method becomes inaccurate when trying to measure the dispersion of devices that have optical characteristics that change rapidly depending upon optical frequency, such as narrow band wavelength division multiplexing (WDM) components. Significant changes in the dispersion or time delay that occur within the frequency range defined by the spacing of the AM sidebands are filtered out. Also, as the two AM sidebands experience differential attenuation due to the filtering effects of these components, an error in the dispersion measurement may occur.
As shown in FIG. 1A, an optical signal having an optical frequency of xcfx890 and two sidebands xe2x88x92xcfx89m and +xcfx89m is passed through a component 5 under test, such as a fiber, which has a transfer function H(xcfx89). The real time delay, xcfx84g, for this signal is equal to the derivative of the phase shift with respect to frequency, as shown in FIG. 1B. An approximate time delay, xcfx84m, can be determined by dividing the difference in phase between the two sidebands by the difference in frequency, which is 2xcfx89m. As shown in FIG. 1B, the slope of the curve at xcfx89o has a negative slope, meaning that the real time delay xcfx84g has a negative value. However, the slope between the points corresponding to the frequencies of the sidebands has a positive slope, meaning that the approximate time delay xcfx84m has a positive value. Accordingly, the approximation used to measure the time delay is inaccurate for fast changes in xcfx84g.
To reduce the errors resulting from differential attenuation, it is possible to reduce the modulation frequency. The reduction in modulation frequency, however, also produces errors. In particular, the reduction causes the phase shifts in the input and the output signals to be impossible to detect, making the phase measurements inaccurate. Since the phase difference, which is directly related to the delay, cannot be measured accurately with the reduced modulation frequency, it is not possible to measure the dispersion accurately.
A method for measuring time delay of a component, consistent with the present invention, measures the phase and magnitude of at least two optical signals output from the component, the at least two optical signals having different modulations. The measured phase and magnitude of the at least two optical signals are adjusted by correcting the measured magnitude of the at least two optical signals for differential attenuation and recovering frequency content in the measured phase of the at least two optical signals. The time delay of the component is determined based upon the adjusted phase and magnitude of the at least two optical signals.